Eye for an eye?

This business with Ronnie Biggs highlights once again the question of what prison is for. On what basis do we lock people up? Unless there are things we are not being told, it would seem unlikely that a sick 79-year-old man poses any real risk to society, so the only reason for keeping him in prison would seem to be revenge, retribution - choose your own word. And you would then have to ask if this is a reasonable and proper use of public money.